High Risk of Muscle Strain in the Rectus Femoris Muscle: Anatomical and Physiological Analysis

Abstract

Objective: Clinical studies reported that the muscle strain of the quadriceps femoris muscles (QF) is most frequent in the rectus femoris (RF) with specific postures. The mechanism underlying the specificity of muscle strain has not been fully understood. We postulated that the physiological and anatomical properties of the muscles constituted a main risk factor of muscle strain. Materials and Methods: Knee extension torque and surface electromyographic (EMG) signals were measured in 18 healthy adults. Anatomical specimens of QF were obtained from 9 cadavers. Results: Isometric knee extensor torque was smallest at knee angle of 10° and gradually increased at larger knee angles of 30°, 60°, and 90°. EMG activities during isometric contraction of the three QF muscles were at a similar level during knee flexion at angles from 10° to 30°, 60°, and 90°, except for a slight non-significant increase at 90° in the vastus lateralis (VL) and the vastus medialis (VM). The four muscles of QF had pennate structure, and their muscle fiber length per total muscle length (FL/TML) was not significantly different. In RF, the muscle fibers were more extended with flexed knee joint and extended hip joint than in the other muscles. Conclusions: The specific high incidence of muscle strain in RF was explained on the basis of physiology and anatomy. In addition, the rationale presented in the present study would be helpful to decrease the incidence of muscle strain by specific exercise to increase the force and extensibility of RF.